Polymorphic materials, having more than one crystalline form, are well known in the art. Several studies have identified these crystalline structures in alternating polyketones and poly(aryl ether ketone ketone) such as Cheng, Z. D. et al, “Polymorphism and crystal structure identification in poly(aryl ether ketone ketone)s”, Macromol. Chem Phys. 197, 185-213 (1996); and Klop. E. A., et. al., “Polymorphism in Alternating Polyketones Studied by X-ray Diffraction and Calorimetry”, Journal of Polymer Science: Part B: Polymer Physics, Vol. 33, 315-326 (1995).
Annealing is known to assist in the development of crystallinity in many semicrystalline polymers. These processes are used in polymer processing and in post forming treatments of polymeric and metallic articles. Typical annealing processes hold the materials above the glass transition of the polymer. These treatments will increase the crystallinity, but may not fully develop the potential level of crystallinity and will not promote a crystal-to-crystal transformation.
US patent application no. 2008/0258330 describes a typical annealing process for poly aryl ether ketone (PAEK) powders. It is believed that this process does not provide the benefits of a highly crystalline material with a narrow melting point range. The described process anneals in a typical fashion “at 20° C. or more above the Tg” and suggests that the temperature of the annealing process should be 30° C. below the melting point of the powder.
We have discovered that heat-treating a semicrystalline or crystallizable polymorphic polymer at a temperature above the melting point(s) of the lower-melting crystalline form(s), but below the highest crystalline melting point, produces a material that maximizes the content of the highest melting crystalline form—unlike in a normal annealing process. The resulting polymer powder has a more uniform melting range, and additionally has improved powder flow and improved durability. In some applications, such as with polyether ketone ketone (PEKK) powders, surprisingly, the crystallinity developed in the powder is retained through the part manufacturing process and results in improved physical properties and reduced deformation than can be developed by post-production annealing.